Models and observations of the growth of the atmospheric boundary layer

Abstract

The evolution of the mixed layer during a clear day can be described with a slab model. The model equations have to be closed by a parameterization of the turbulent kinetic energy budget. Several possibilities for this parameterization have been proposed. In order to assess the practical applicability of these models for the atmosphere, field experiments were carried out on ten clear days in 1977 and 1978. Within the accuracy of the measurements the mixed-layer height in fully convective conditions (at noon on clear days) is well predicted taking a constant heat flux ratio -0~~ = 0.2&v,. In the early morning hours mechanical entrainment is also important. Good overall results are obtained with the entrainment formulation -Bw,, = 0.2&v, + Su?T/gh. Only large differences in the entrainment coefficients lead to significantly different results. Making the entrainment model more complex does not lead to substantial improvement. The mean potential temperature in the mixed layer is reproduced within 0.5 "C. This temperature is insensitive to the choice of a particular entrainment formulation and depends more on the surface heat input and the temperature gradient in the stable air aloft.

Effects of shear in the convective boundary layer: analysis of the turbulent kinetic energy budget

“…By analyzing the turbulence kinetic energy (TKE) budget, it is possible to obtain a parametric expression which depends only on scaling parameters. These expressions have been evaluated with observations (Artaz and André 1980, Dubosclard 1980, Driedonks 1982, Culf 1992, Betts and Ball 1994, Angevine et al 1998, Hägeli et al 2000. By comparing the parameterized boundary layer depth or entrainment flux ratio against observations, the authors of some of these studies pointed to the importance of taking shear and dissipation contributions into account in the parameterizations.…”

Sheared convective boundary layers: turbulence kinetic energy and entrainment dynamics

“…The entrainment dynamics of sheared CBLs that are driven by both convection and wind shear, turned out to be a more challenging issue. Based on an extensive atmospheric observations Driedonks (1982) found that wind shear can significantly enhance the diurnal growth of fair weather CBLs, which rather well corresponded with prediction of a simple bulk model. However, as typical for atmospheric observations, uncertainties remained very high.…”

“…Deardorff et al, 1969;Deardorff and Wilis, 1985;Fedorovich et al, 2001b) or atmospheric observations (e.g. Stull, 1976a;Driedonks, 1982). However, if appropriate data are available, which is potentially always the case when using LES or DNS, we find this practice methodologically inconsistent.…”

“…65 Driedonks (1982) demonstrated that for a typically very patchy (in space and time) set of atmospheric observations, which also comes with a high statistical uncertainty, the analysis of the temporal evolution of the CBL mean state is the best analytical strategy. This procedure seems to have influenced many CBL studies afterwards, although Driedonks (1982) also explicitly mentioned that these arguments do not hold for more complete data sets with sufficient statistical representativity, as e.g.…”

“…This procedure seems to have influenced many CBL studies afterwards, although Driedonks (1982) also explicitly mentioned that these arguments do not hold for more complete data sets with sufficient statistical representativity, as e.g. provided by any reasonable LES or DNS experiment.…”